Various methods of deposition of metals on ceramic bases are known in the art. Included among the most recent state-of-art methods are those disclosed in our U.S. Pat. Nos. 4,358,506; 4,396,677; 4,374,903, and 4,376,806. These methods have proven to be major advances in the chemical deposition of metals on ceramics. Nonetheless, these methods have been further improved by the invention as disclosed herein.
The methods disclosed in the above-mentioned patents comprised reacting a coated substrate in a CO or some other atmosphere containing CO as the reductant at temperatures ranging from about 850.degree. C. to about 1000.degree. C. to bond the coating to the substrate. The discovery of improvement has been occasioned by the following. First, at the higher reaction temperatures the decomposition temperatures are approached for some of the disclosed substrates. Conversely, the exposure time of the substrates must be very limited, or very rapid surface heating must be achieved. For some of the materials, due to phase changes, thermal expansion, etc., a specific rate of heating must be maintained, e.g. ferrites. Secondly, in that some of the disclosed substrates are oxides, reduction in a gas such as CO or a gas that contains or produces CO affects the substrate. The reaction of an oxide substrate in a reducing atmosphere at temperatures ranging from 600.degree. C. to 1000.degree. C. are generally well known and therefore need not be discussed herein. The present invention, however, eliminates the first of the above-mentioned problems and decreases the significance of the second, especially for substrates that are affected by carbon monoxide at lower temperatures. Moreover, preheating in an inert atmosphere, e.g. nitrogen helium and the like, also minimizes the problems with carbon monoxide attack on the substrate.